Our previous studies have confirmed that there is a more detailed organization among group Ia spindle afferents and their homonymous alpha motoneurons than the simple concept that all the afferent neurons project rather homogeneously to all of the motor neurons. In fact, recent work from other laboratories has demonstrated that the motor nucleus may be divided into at least three subsets of functionally differentiable motor units and that a given synaptic input (tested from a variety of sources) may affect differently the various subsets in both a quantitative and a qualitative manner. On the other hand, work in other laboratories has suggested that the motor nucleus of a given muscle is organized as a continuum of motor units whose activities are chosen according to a size principle in which cell geometry predominates in the selection of units to be either excited or inhibited. The latter principle has much evidence accumulated in its favor, but some exceptions have been noted. We have subdivided the peripheral distribution of muscle afferents and motor axons by dissecting a muscle nerve into a number of naturally occurring intramuscular branches such that they could be stimulated separately. Intracellular records from homonymous alpha motoneurons indicate that the anatomically separate branches project their group Ia depolarization more strongly to one motoneuron than another, and not at all to others. In the present work, it is proposed to use the above-mentioned method of peripherally subdividing a muscle nerve to access separate sets of synaptic inputs to the motor nucleus and to assess with intracellular recording techniques their connectivities from both an anatomical and a functional point of view. Our primary objective is to elucidate the principles governing segmental sensory-motor organization. Antidromic activation of the separate subsets of motor axons will be used also to study the organization of Renshaw inhibition within the motor nucleus. It is felt that these studies will help to clarify some of the current questions regarding applicability of the size principle, to explain the basis for intramuscular localization of the stretch reflex, and to enhance our understanding of the functional organization of sensory-neuromuscular systems.